There's some info on updating Certbot in this thread:
But the short of it is that Debian 8 is Just Too Old.
https://www.debian.org/News/2020/20200709
Your best bet is to upgrade to an operating system that still gets patches. Your second-best bet is to switch from certbot to some other ACME client which is easy to install on old systems (I'd suggest something from the Bash or Go sections of the client list), but keep in mind that if your system isn't getting security patches then even though you have a certificate your data and connections may be at risk from unpatched vulnerabilities.